Charge-transfer switched-capacity filter

ABSTRACT

A switched capacity filter having capacities formed by MOS technology on a semiconductor substrate. The connection between two capacities whose first plates are formed by the semiconductor substrate are periodically connected by providing transfer of charges in the substrate on which these two capacities are integrated. The external or other plate of each capacity receives, the input voltage, or a reference voltage, or the surface potential under another capacity which is provided by a reinjection and reading device formed from a diode and a voltage follower stage.

BACKGROUND OF THE INVENTION

The present invention relates to a charge-transfer switched-capacity filter.

Switched-capacity filters are known particularly from two articles in the American review "IEEE Journal of Solid-State Circuits", volume SC-12, No. 6, Dec. 1977, pages 592 to 608.

Switched-capacity filters comprise generally an amplifier associated with a network of resistors and capacities in which each resistor is formed by the series connection of two MOS switches and by a capacity between the common point between these switches and a reference voltage.

SUMMARY OF THE INVENTION

The present invention relates to a switched-capacity filter in which the capacities are formed by MOS technology and the MOS switches are formed by grids astride the MOS capacities from which they are separated by an oxide layer.

One plate of each MOS capacity is then formed by the semiconductor substrate on which this capacity is integrated and the connection between two MOS capacities whose plates formed by the substrate are periodically connected, depending on the electrical diagram of the filter, is provided by charge transfer in the semiconductor substrate on which these two capacities are integrated and results in the establishment of the same surface potential under these two capacities. This is why the filter of the invention is called a charge-transfer switched-capacity filter.

The other plate of each MOS capacity, which is then external to the substrate, receives, depending on the point where it is connected in the electrical diagram of the filter, either the input voltage of the filter E, or a DC voltage V_(G) taken for reference. In the case where the external plate is connected in the electrical diagram, periodically or permanently, and directly or through the amplifier, to the plate formed by the substrate of another capacity, this external plate receives through a reading and reinjecting device the surface potential under this other capacity.

The present invention allows a filter whose electrical diagram is commonplace and only comprises resistors and capacities associated with an amplifier to be transformed into a charge-transfer switched-capacity filter. Thus, charge-transfer switched-capacity filters of any order may be obtained by placing in series filters of the first and second orders.

Charge-transfer switched-capacity filters present, particularly with respect to known switched-capacity filters, the advantage of not comprising any parasite capacities on the capacities of the network associated with the amplifier. Thus, low values may be used for the capacities of the network and the cost thereof may be reduced and the compactness increased.

DESCRIPTION OF THE DRAWINGS

Other objects, characteristics and results of the invention will become clear from the following description, given by way of non-limiting example and illustrated by the accompanying figures which represent:

FIG. 1, the electrical diagram of a high-pass filter of the second order;

FIG. 2, the diagram of a switched-capacity filter corresponding to the filter of FIG. 1;

FIG. 3, one embodiment in accordance with the invention of a charge-transfer switched-capacity filter corresponding to the filter of FIGS. 1 and 2;

FIG. 4, the electrical diagram of a low-pass filter of the second order;

FIG. 5, the diagram of the switched-capacity filter corresponding to the filter of FIG. 4;

FIG. 6, one embodiment in accordance with the invention of a charge-transfer switched-capacity filter corresponding to the filter of FIGS. 4 and 5;

FIG. 7, part a to e, phase diagrams of signals able to be applied to the filters of the invention; and

FIG. 8, another embodiment in accordance with the invention of a charge-transfer switched-capacity filter corresponding to the filter of FIGS. 1 and 2.

In the different figures, the same references designate the same parts, but for the sake of clarity, the sizes and proportions of the different parts have not been respected.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the electrical diagram of a high-pass filter of the second order.

This filter is formed from two capacities C₁ and C₂ in series with the input of an amplifier 1 with gain G. A resistor R₁ is connected between the input of the amplifier and ground, whereas another resistor R₂ ensures looping of the filter by connecting the output of the amplifier to the common point between capacities C₁ and C₂.

We will call E the input voltage of the filter and V_(S) its output voltage.

FIG. 2 shows the diagram of the switched-capacity filter corresponding to the filter of FIG. 1.

The resistors R₁ and R₂ of the filter of FIG. 1 are formed by placing in series two MOS switches, respectively I₁, I₂, and I₃, I₄, receiving control signals φ_(A) and φ_(B), and by a capacity, respectively C₃ and C₄, between the point common to both switches and ground.

In FIG. 2 there has been shown symbolically by a few dots, which plate of capacities C₁ to C₄ would be formed by the semiconductor substrate in the charge-transfer switched-capacity filter of the invention. In the filter of the invention, there will then be formed by the substrate the plate of capacity C₁, other than the one which receives the input voltage E in accordance with the electrical diagram, the plates of C₃ and C₄ other than those connected to ground according to the electrical diagram and finally the plate of capacity C₂ which is connected to amplifier 1.

FIG. 3 shows one embodiment in accordance with the invention of a charge-transfer switched-capacity filter corresponding to the filter of FIGS. 1 and 2.

The capacities of the filter are formed by MOS technology, i.e. they are formed by a metal electrode, separated by an oxide layer (which is not shown in the figure for the sake of clarity) from a semiconductor substrate 2, which is generally silicon. One of the plates of each MOS capacity is then formed by the substrate on which it is integrated.

The MOS switches of the filter are formed by grids astride the MOS capacities from which they are separated by an additional oxide layer (which is not shown in the figure also for the sake of clarity).

When a high potential is applied to one of these switches, a charge-transfer connection is effected between the two capacities adjacent this switch. The same surface potential is finally established under these two capacities.

On the same semiconductor substrate we find integrated one after the other, depending on the charge-transfer direction indicated by an arrow, the capacities C₄ and C₁ separated by switch I₃ controlled by the signal φ_(B). In the diagram of FIG. 2, the plates formed by the substrate of capacities C₁ and C₄ are periodically connected by MOS switch I₃ controlled by φ_(B). These capacities are then integrated on the same semiconductor substrate and the connection therebetween takes place by charge-transfer in the substrate through I₃. Since the other plate of capacity C₄ is connected, according to the diagram of FIG. 2, to ground, it receives a DC voltage V_(G) taken for reference. The other plate of capacity C₁ receives, as shown in FIG. 2, the input voltage of the filter E. Capacities C₂ and C₃ whose plates formed by the substrate are periodically connected by switch L₁ controlled by φ_(B) in FIG. 2 are integrated on the same semiconductor substrate 2 as that which bears C₄ and C₁ but are isolated from the zone of the substrate which carries C₄ and C₁.

The other plate of C₃ connected to ground in FIG. 2 receives the voltage V_(G).

The other plate of C₂ which is connected according to the diagram of FIG. 2 to the plate formed by the substrate of C₁ receives the surface potential under C₁ through a reading and reinjection device.

In the case of FIG. 3, the reading and reinjection device is formed by a voltage-follower stage whose input is connected to a diode D₁ diffused in the substrate under C₁ and whose output is connected to the external plate of C₂.

The voltage-follower stage may be formed, as shown in FIG. 3, by two MOS transistors, an enrichment transistor T₁ and a depletion transistor T₂, in series between a supply voltage V_(DD) and ground. The input takes place on the grid of T₁ connected to V_(DD) and the output takes place on the electrode common to T₁ and T₂, whereas the grid of T₂ is connected to ground.

In FIG. 2, it can be seen that the plate formed by the substrate of the capacity C₂ is connected through the gain amplifier G and a switch I₄ controlled by the signal φ_(A) to the plate formed by the substrate of capacity C₄.

In the case of FIG. 3, that is achieved by means of a charge-injection and reading device formed by a diode D₂ injected under C₂ and a voltage-follower stage connected to the input of the gain amplifier G. The output of the amplifier is connected to a charge-injection diode D₄, diffused in the same substrate as C₄ and C₁ and downstream, in the charge-transfer direction, of C₄ from which it is separated by switch I₄ controlled by φ_(A).

The voltage-follower stage is generally included in gain amplifier G and, in FIG. 3, only this amplifier is shown.

In FIG. 2, it can be seen that the plate formed by the substrate of C₃ is periodically connected through a switch I₂ controlled by φ_(A) to ground.

In the case of FIG. 3, that is achieved by disposing after C₃ on the same substrate a switch I₂ controlled by φ_(A) and a diode D₃ permanently connected to a reference potential V_(ref) which may be the ground of the filter.

There will now be described the transfer function of the filter of the invention, shown in FIG. 3.

It should first of all be noted that the clock signals φ_(A) and φ_(B) applied to the MOS switches of this filter are shown in FIGS. 7a and 7b. The signals φ_(A) and φ_(B) vary substantially as a square wave between a low level and a high level with a period T. Furthermore, φ_(A) and φ_(B) are not simultaneous at the high level. We will call t_(A) the time when φ_(A) is at the high level and t_(B) the time when φ_(B) is at the high level.

The same input voltage E is maintained on C₁ between two successive times t_(B) and changes at times t_(B). At time t_(A), φ_(A) is at the high level and φ_(B) at the low level. The surface potential under C₄ and under C₃ is written:

    φ.sub.S4 (t.sub.A)=G.φ.sub.S2 (t.sub.B -T) and φ.sub.S3 (t.sub.A)=V.sub.ref.

The conservation of the total quantity of charges on the one hand under C₄ and C₁ and on the other hand under C₂ and C₃ between times t_(A) and t_(B) may furthermore be written:

    C.sub.4 ]V.sub.G -Gφ.sub.S2 (t.sub.B -T)]+C.sub.1 [E(t.sub.B -T)-φ.sub.S1 (t.sub.B -T]=C.sub.4 [V.sub.G -φ.sub.S1 (t.sub.B)]+C.sub.1 [E(t.sub.B)-φ.sub.S1 (t.sub.B)]

    C.sub.2 [φ.sub.S1 (t.sub.B -T)-φ.sub.S2 (t.sub.B -T)]+C.sub.3 (V.sub.G -V.sub.ref)=C.sub.2 [φ.sub.S1 (t.sub.B)-φ.sub.S2 (t.sub.B)]+C.sub.3 [V.sub.G -φ.sub.S2 (t.sub.B)]

Passing into the Z plane and eliminating φ_(S1) from these two relationships, we obtain: ##EQU1## where A, B and C are constants which are expressed as a function of G and C₁, C₂, C₃, C₄.

It can then be seen that the transfer function at Z obtained is that of a high-pass filter.

FIG. 4 shows the electrical diagram of a low-pass filter of the second order, of the Sallen-Key type, dual with that shown in FIG. 1.

This filter comprises two resistors R₃ and R₄ in series with the input of amplifier 1 with gain G. A capacity C₅ is connected between the input of the amplifier and ground. Finally, a capacity C₆ between the output of the amplifier and the common point between R₃ and R₄ ensures looping of the filter.

FIG. 5 shows the diagram of the switched-capacity filter corresponding to the diagram of FIG. 4.

Resistors R₃ and R₄ are formed by placing in series two switches, respectively I₅, I₆ and I₇, I₈ and by a capacity respectively C₇ and C₈ between the point common to the switches and ground.

There is shown symbolically by a few dots that the plates of C₅, C₇, C₈, other than those connected to ground in accordance with the electrical diagram, would be formed by the semiconductor substrate of the filter of the invention. Similarly, it has been shown symbolically that the plate of C₆ other than the one connected to the output of the amplifier would be formed by the semiconductor substrate.

FIG. 6 shows one embodiment in accordance with the invention of a charge-transfer switched-capacity filter corresponding to the filter of FIGS. 4 and 5.

We find aligned on the same substrate, in the charge-transfer direction as shown by an arrow, the capacities C₇, C₆, C₈, C₅ and a reading capacity C_(l). Each of these capacities is separated from the next one by a switch I₆, I₇, I₈, I₉.

Switch I₇ is controlled by φ_(A), switches I₆ and I₈ are controlled by φ_(B) and finally switch I₉ is controlled by a signal φ_(L).

The external plate of capacities C₇ and C₈ is connected directly to V_(G). The external plate of C₅ is periodically connected to V_(G) through an MOS transistor T₃ controlled by the signal φ_(B).

The input voltage E is applied to a diffused diode D₅, downstream of C₇, from which it is separated by a capacity brought up to V_(G) and by a switch I₅ controlled by φ_(A).

In the diagram of FIG. 5, the plate formed by the substrate of C₅ is connected through the amplifier to the external plate of C₆. That is achieved in FIG. 6 by a reinjection and reading device connected between C₅ and the amplifier, the output of the amplifier being connected to the external plate of C₆.

The reinjection and reading device shown in FIG. 6 comprises:

the reading capacity C_(l) separated from C₅ by switch I₉ controlled by φ_(L) ;

an MOS transistor T₄ connected between the external plate of C₅ and ground and controlled by φ_(L) ;

a stage formed by an MOS transistor T_(L) controlled by a signal φ_(P) and connected between the external plate of C_(l) and the input of the amplifier at a point A. To point A are also connected a capacity C_(A) which is also connected to ground and a transistor T₆ controlled by a signal φ_(C), and connected to the supply voltage V_(D) ;

finally, a transistor T₅ controlled by φ_(B) is connected between the external plate of C_(l) and ground.

In FIGS. 7c, 7d and 7e, there is shown after φ_(A) and φ_(B), the clock signals φ_(C), φ_(L) and φ_(P). These signals, like φ_(A) and φ_(B), vary substantially as a square wave between a low level and a high level with a period T.

The signals φ_(A), φ_(B), φ_(C) and φ_(L) are never simultaneously at the high level. In chronological order, φ_(A), φ_(C), φ_(L) are at the high level then φ_(B) and then again φ_(A), φ_(C) . . .

Signal φ_(P) is at the high level from the time when φ_(C) passes to the high level to the time when φ_(L) passes to the low level.

Finally, we will call t_(C) and t_(L) the times when φ_(C) and φ_(L) are at the high level.

There will now be explained the operation of the reinjection and reading device.

At time t_(C), φ_(C) and φ_(P) are at the high level. Transistor T₆ conducts and causes capacity C_(A) to be charged to a level V_(AO) =V_(D) such that transistor T_(L), whose grid receives φ_(P), is biased to saturation. The reading capacity C_(l) then receives from T_(L) on its external plate a voltage equal to V.sub.φP -V_(TL), where V.sub.φP represents the high level of the signal φ_(P) and V_(TL) the threshold voltage of T_(L).

At time t_(L), φ_(L) and φ_(P) are at the high level. Transistor T₄ conducts and connects the external plate of C₅ to ground. The inversion charge present under C₅ is transferred through switch I₉ which is conducting under C_(L).

The external plate of C_(L) is maintained at a constant potential by T_(L) which is still saturated, which causes capacity C_(A) to be discharged and the potential at point A to be modified.

The potential of point A a time t_(L) is therefore written: ##EQU2## where Q₅ (t_(B) -T) represents the charge present under C₅ at the end of the preceding cycle, by making the approximation that a space charge under C_(L) at time t_(L) is not very different from that existing under C₅ at time t_(B) -T, which is justified particularly if C₅ =C_(L), which is the case in FIG. 6.

The maintenance of a constant potential on the external plate of C_(L) during the transfer of the charges to be read under this capacity allows reading of the surface potential under C_(L), and so under C₅, at point A.

To assure proper operation of the filter, amplifier 1 must comprise a continuous-level translator stage so that: ##EQU3##

In fact, at time t_(A), a common-surface potential is established between C₆ and C₈. For it to be possible to establish this common-surface potential, it is preferable for the voltage applied to C₆ in the absence of a reading signal to be the same as that applied to C₈, i.e. the reference voltage V_(G).

At time t_(B), φ_(B) is at the high level and transistor T₅ connects the external plate of C_(L) to ground, which causes the return of the charges read at time t_(L) under C₅.

By writing the conservation of the charges at the different times on the capacities of the filter, we obtain in the Z plane the transfer function of the filter which is written: ##EQU4## where A, B, C, D and G are constants which are expressed as a function of the values of C₅, C₆, C₇ and C₈, and where β=t_(B) -t_(L).

The filter obtained has a low-frequency response similar to that of a filter of the second order. At the Nyquist frequency, there exists a real pole, but it does not interfere with the low-frequency operation.

FIG. 8 shows another embodiment in accordance with the invention of a charge-transfer switched-capacity filter corresponding to the filter of FIGS. 1 and 2.

FIG. 8 differs from FIG. 3 solely by the reinjection and reading devices used. The devices used in FIG. 8 are the same as the one which is used in FIG. 6 for the low-pass filter.

Thus, the surface potential under C₁ and C₂ is read thanks to a transfer of charges under two reading capacities CL₁ and CL₂ separated from C₁ and C₂ by switches I₁₀ and I₁₁ receiving φ_(L). As in the case of FIG. 6, the transfer of the charges under the reading capacities CL₁ and CL₂ is provided by MOS transistors connected between the external plate of C₁ and C₂ and ground and controlled by φ_(L) and the return of the charges under C₁ and C₂ is provided by MOS transistors connected between the external plate of CL₁ and CL₂ and ground and controlled by φ_(B). These transistors are not shown in FIG. 8.

It would be similarly possible to use the reinjection reading device of FIG. 3 for the construction of a charge-transfer switched-capacity filter corresponding to FIGS. 4 and 5.

Finally, it will be readily understood that the diagrams of FIGS. 1 and 4 are only given by way of example and that the invention is applicable to filters whose electrical diagram is commonplace and only comprises resistors and capacities associated with an amplifier. 

What is claimed is:
 1. A charge-transfer switched-capacity filter, comprising an amplifier associated with a network of resistances and capacities in which each resistance is formed by two MOS switches in series and with a capacity between the common point of these switches and ground; said capacities are formed by MOS technology and said MOS switches are formed by control electrodes adjacent the MOS capacities and from which they are separated by an oxide layer; one plate of each capacity being formed by the semiconductor substrate on which it is integrated, and connection between two capacities whose plates are formed by the substrate are periodically connected by transfer of charges in the semiconductor substrate on which said two capacities are integrated, and results in the establishment of the same surface potential under said two capacities; the other external to the substrate plate of at least one of the capacities receiving input voltage of the filter, and of at least another of the capacities receiving a DC voltage taken for reference.
 2. A filter according to claim 1 further comprising a reinjection and reading device between one external plate of a capacity termed a reading capacity, and the plate formed by the substrate of another capacity for reading the surface potential under said other capacity.
 3. The filter as claimed in claim 2, wherein said reinjection and reading device is formed by a voltage-follower stage whose input is connected to a diode formed in the semiconductor substrate on which a capacity is integrated and whose output is connected to the external plate of another capacity.
 4. The filter as claimed in claim 3, wherein said voltage-follower stage is formed by two MOS transistors in series between a supply voltage and ground, one of said transistors being an enrichment mode, and the other a depletion mode, and both having a control electrode, the input of the stage taking place on the control electrode of the MOS enrichment transistor connected to the supply voltage and the output of the stage taking place on the electrode common to the two transistors, whereas the control electrode of the depletion MOS transistor connected to ground is also connected to ground.
 5. The filter as claimed in claim 2, wherein the reinjection and reading device comprises:an MOS reading capacity integrated on the substrate following the capacity whose surface potential is to be read, an MOS switch ensuring the connection between these two capacities; an MOS transistor connected between the external plate of the capacity to be read and ground, this transistor being enabled while the switch allows the transfer of the charges from the capacity to be read to the reading capacity; a stage, connected to the external plate of the reading capacity ensuring the maintenance of a constant potential on the external plate of the reading capacity during transfer of the charges under this capacity, this stage being connected, directly or through said amplifier, to the external plate of another capacity.
 6. The filter as claimed in claim 5, wherein said stage is formed by an MOS transistor operating under saturation connected between the external plate of the reading capacity and a predetermined point and by a capacity between said point and ground, this capacity being periodically charged before the arrival of the charges under the reading capacity and discharged by the arrival of the charges under the reading capacity.
 7. The filter as claimed in claim 6, wherein an MOS transistor connected between the external plate of the reading capacity and ground ensures periodically the return of the charges from the reading capacity to the capacity to be read and wherein an MOS transistor connected between said point and a supply voltage ensures periodically the charging of the capacity connected to said point.
 8. The filter as claimed in claim 7, wherein a continuous-level translator stage is connected to said point and translates the continuous level of the signal obtained at said point from v_(AO), which is the voltage at the terminals of the capacity connected to said point before the arrival of the charges under the reading capacity, to V_(G), which is the DC voltage taken for reference.
 9. The filter as claimed in claim 8, whose electrical diagram is formed by two capacities C₁ and C₂ in series with the input of the amplifier, a resistor between the input of the amplifier and ground and a looping resistor formed from two switches and a capacity C₄ between the output of the amplifier and the point common to capacities C₁ and C₂, wherein the plate formed by the substrate of capacity C₁ is connected according to the electrical diagram to the external plate of capacity C₂, and wherein a reinjection and reading device is connected between the plate formed by the substrate of C₁ and the external plate of C₂.
 10. The filter as claimed in claim 9, wherein the plate formed by the substrate of C₂ is connected, according to the electrical diagram, through the amplifier and a switch to the plate formed by the substrate of C₄ and wherein a reinjection and reading device is connected between the plate formed by the substrate of capacity C₂ and the amplifier, the amplifier being connected to a charge-injection diode integrated in the same substrate and downstream in the charge-transfer direction of capacity C₄ from which it is separated by a switch.
 11. The filter as claimed in claim 8, whose electrical diagram is formed from two resistors in series with the input of the amplifier, a capacity C₅ between the input of the amplifier and ground and a looping capacity C₆ between the output of the amplifier and the point common to the two resistors, wherein the plate formed by the substrate of capacity C₅ is connected, in the electrical diagram, through the amplifier to the external plate of capacity C₆ and wherein a reinjection and reading device is connected between the plate formed by the substrate of C₅ and the input of the amplifier which is connected to the external plate of capacity C₆. 